The invention relates to active vibration control.
As used herein, the term "vibration" includes sound or noise, and the invention is particularly concerned with active noise control.
In passenger compartments of cars (automobiles), where a significant noise component is harmonically related to the rotation frequency of a reciprocating engine used to drive the car, the sound levels at low frequencies in such enclosures are difficult to attenuate using conventional passive methods and can give rise to subjectively annoying "boom". A method of actively attenuating a simple sound field by introducing a single secondary sound source driven so that its output is in antiphase with the original ambient noise is described in general terms by B. Chaplin in "The Chartered Mechanical Engineer" of January 1983, at pages 41 to 47. Other discussions are to be found in an article entitled "Active Attenuation of Noise-The State of the Art" by Glenn E. Warnaka at pages 100 to 110 in Noise Control Engineering, May-June 1982 and in Internoise 83 Proceedings, pages 457 to 458 and 461 to 464, and Internoise 84 Proceedings, pages 483 to 488. Particular methods and apparatus are also described in British Patent Specification Nos. 1,577,322 and 2,149,614.